This invention relates to a front projection screen for use in displaying an image projected from an image source. More particularly, the invention relates to a front projection screen employing a light concentrating lens array and light-reflecting and absorbing elements behind the array, the lens array concentrating projected light from an image source onto the reflecting elements, the reflecting elements distributing the light back in to an audience field, and the absorbing elements trapping ambient light.
Front projection screens are useful in a variety of applications, for example, television, motion picture and other video and film media displays. In such applications, an image source in front of the screen projects an image forward along a central projection axis to the screen, which displays the image to viewers in an audience field, also located in front of the screen.
A problem encountered in these front projection systems is that the image contrast is often reduced by the presence of ambient light. This is particularly troublesome in those applications in which a dedicated projection room is not practical, for example, projection television for the consumer market. In such cases, the source and character of ambient lighting is generally unpredictable and thus uncontrollable.
It is known to increase the image contrast of front projection screens by concentrating the projected light into a multiplicity of picture elements, or "pixels", and by providing an array of reflective areas on a rear surface of the screen to reflect the pixels back through the front surface and into the audience space. Ambient light which enters the screen at oblique angles to the projection axis generally falls on light absorbing areas surrounding the light reflecting areas, and is consequently absorbed. See U.S. Pat. Nos. 1,942,841; 4,298,246; 3,704,055; and 4,606,609.
In U.S. Pat. Nos. 4,298,246 and 1,942,841, the front surface is made up of a two-dimensional array of convex lenslets which focus the projected image behind the reflecting areas of the rear surface of the screen, resulting in some spreading of the image as it is reflected back into the audience field.
In U.S. Pat. Nos. 4,606,609 and 3,704,055, the screen is made up of a plurality of optical cells, each having a front convex lens surface, and a rear surface contoured to collimate the reflected light into a narrow beam. Spreading of the image is accomplished by light diffusing areas on the front lens surfaces located in the path of the reflected light, which diffuse the beams into the audience field.
In all of the above arrangements, optimum performance is dependent upon holding a relatively close tolerance on screen thickness, as well as the registration between the front and back surfaces, both needed to insure that the focal points of the front surface lenses fall at the desired location relative to the rear reflecting surface.
In addition, these arrangements are designed with the assumption that the projected light is parallel to the projection axis and thus normally incident on the screen surface. For a flat screen, this is generally true only in a central portion of the screen near the projection axis. With increasing distance away from the projection axis, the angle of incidence of the projected light increases, increasing the likelihood that the pixels will be partly or totally intercepted by the light absorbing areas.
In U.S. Pat. No. 3,704,055, it is suggested that this problem be solved by inclining the optical cells so that their optic axes are coincident with the projected rays, for example, by spherically contouring the screen. However, such contouring has been found in practice to be a difficult task, tending to frustrate economical mass production of these screens.
An alternative to spherically contouring the screen would be to provide a circular Fresnel lens in front of the screen to collimate the projected light so that it is normally incident on the light concentrating lens surfaces. However, the presence of such an additional lens would not only reduce screen efficiency, primarily through increased reflection losses, but also would decrease screen contrast, through increased reflection of ambient light, as well as increase the structural complexity of the screen, adding to fabrication costs. Thus, Fresnel lenses, while widely used in rear projection screens, have not been commercially used in front projection screens.
Accordingly, it is an object of the invention to provide a front projection screen of the type which concentrates projected light onto an array of light reflecting areas bordered by light absorbing areas for image contrast, which screen does not rely upon a front surface array of lenses to concentrate the light.
It is another object of the invention to provide such a front projection screen which does not rely for optimum performance upon control of screen thickness and registration between arrays on the front and back surfaces of the screen.
It is still another object of the invention to provide such a front projection screen having a single transmissive layer with a front surface available for a light collimating or other lens element.
It is still another object of the invention to provide such a front projection screen which can be readily mass-produced.